The present invention relates to tires. More particularly, it concerns the design of the beads of tires.
The purpose of the beads of tires is known; it is to assure proper mounting of the tire on the rim. For this purpose, the carcass cables all reach the lower part of the bead where they are firmly anchored in order that the carcass can withstand the stresses in operation.
Very recently, a new type of bead has been proposed in EP patent application 0582196 (corresponding U.S. application Ser. No. 08/331,373. This bead does not have the customary turn-up of the carcass around a bead wire. Instead, at the place of the anchoring, the carcass reinforcing elements are arranged in one or more alignments. Considering the arrangement of the assembly of these elements in space, they approximate the shape, within each alignment, of a partial conical frustrum the axis of which coincides with the axis of rotation of the tire. The carcass reinforcing elements are bordered laterally by at least one pile of circumferential reinforcing elements produced for instance by helical winding. Furthermore, a suitable connecting rubber mix assures the transmission of forces between these reinforcing elements which are directed perpendicular to each other.
The tests carried out by the applicant have shown that such a bead structure excellently withstands the stresses encountered in use, even severe use, in both passenger car tires and tires for other applications. In addition to the stresses to which the tire is subjected during operation, the tire must furthermore be capable of experiencing an indeterminate number of removals followed by remounting in order to continue the use thereof.
It is known that when the tire is mounted on a wheel, the greater the clamping of the bead the less the tendency of the tire to leave the rim. It may be recalled that the clamping is the force of compression of the rubber located in the radially lowermost part of the bead, developing a pressure on the radially outer surface of the seat of the corresponding rim. A certain level of clamping is necessary in order to be able to transmit a braking or driving torque between the rim and the tire. The clamping depends not only on the properties of the tire itself (geometry of the bead, rigidity of the materials used in it) but also the geometry of the rim itself.
It is also known that the greater the clamping the greater will be the difficulties in mounting and/or dismounting the tire. The dismounting, in particular, involves applying a rather large force (a function of the clamping on the rim) on the bead at the level of the flange of the rim or just above it. This force is directed parallel to the axis of rotation and is always applied locally by a push bar or a lever. These tools apply a deformation on the bead of the tire. This deformation may be very substantial. This is the first phase of the dismounting, the purpose of which is to unseat the bead, that is to say cause it to leave its seat, by removing it from the rim flange. During this first phase, the bead of the tire is subjected to local but very substantial stretching forces.
Thereupon, levers are generally used in order to force the bead over the rim flange. In fact, in the case of tires formed in a single piece (which is the general case for tires for passenger cars and vans), the shape of the rim is designed to permit mounting and dismounting due to an ovalizing of the bead without increasing its perimeter. This substantially determines the design of the central mounting well and of the flanges laterally bordering the rim and defining the mounting position of the bead. During this second phase, the bead undergoes an overall deformation which is far less penalizing than the stresses occurring during the first phase.
The designer of the tire seeks to achieve a good compromise between safety (low sensitivity to unseating), in particular, by controlling the clamping, and ease of mounting/dismounting. Satisfaction of these requirements (operating stresses and stresses upon mounting and dismounting) which are somewhat contradictory and the desire to simplify the manufacture and limit the weight of material are objects of the present invention.
An object of the invention is to improve the ability of the bead structure described in EP Patent 0582196 referred to above to undergo dismountings, even effected under less careful conditions, in particular with unsuitable tools. In this case, during the first phase of the dismounting, the point of the bead, retained by a hump, is subjected to a rotation centered substantially on said point (see FIG. 3) because this type of bead is rather flexible in rotation in a radial plane. If this rotation extends so far as a swinging of the bead, as shown in FIG. 3, a part of the helices of the circumferential cable 2 experiences very extensive elongation. This elongation may reach about 3% in the case of the lowest helices 40 of the axially outer pile 4. In the extreme cases, this elongation is combined with a local deformation due to the pressure of the dismounting tool, the dismounting may cause the breaking of the cable in one or more helices.
In particular, the present invention is directed at permitting a very large number of dismountings, with possible reuse of the tire, without sacrificing the performance of the tire in use.
In accordance with the invention, the tire has sidewalls which terminate in beads, the beads being designed to be mounted on a rim, said tire comprising a carcass reinforcement which passes into the side walls and joins the beads, one at least of said beads comprising:
carcass reinforcing elements which extend from the radially bottom part of the bead towards the sidewall,
at least one pile of circumferential cables laterally bordering the carcass reinforcing elements, said circumferential cables having an operational elongation rate Af=Ae+Ap of more than 4%,
a connecting rubber mix arranged between the circumferential cables and the carcass reinforcing elements.
FIG. 4 shows a stress-strain curve. There can be noted first of all an elongation As which is the specific elongation of the xe2x80x9ccablexe2x80x9d effect. This elongation represents a clamping together of the wires of the cable before said wires are even stressed in traction. In the diagram, there can then be noted a plastic elongation Ap and finally an elastic elongation Ae.
For the description of the invention, we are introducing here the concept of operational elongations rate Af=Ae+Ap. This operational elongation rate does not include the specific elongation As of the xe2x80x9ccablexe2x80x9d effect.
As for the maximum stress, it is determined by the following formula:   Rm  =      Fmp          M      /      L      
in which Fm is the maximum force, xcfx81 is the density of the material in question (7.8 g/cm3 for the steel used), and M/L is the linear weight of the cable used.
The maximum stress Rm is preferably greater than 2000 MPa and even advantageously greater than 2200 MPa, which makes it possible to construct a bead which is as light as possible, lightness being itself a performance factor and contributing to limiting the cost of the tire.
The invention will be fully understood from a reading of the following description, illustrated by means of the accompanying figures, and, in non-limitative manner, a specific embodiment of a tire bead.